Help With Iptables

technick · 04-03-2007, 10:39 AM

This is a fairly simple problem I just can't figure out. I need to forward all inbound connections to port 25 (smtp) to another ip address on the same port.

I keep 25 closed on my outbound mail server and some spam filters try to connect back to verify that I am not a bot I guess.

This is what my table looks like right now.

Code:

# Firewall configuration written by system-config-securitylevel
# Manual customization of this file is not recommended.
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:RH-Firewall-1-INPUT - [0:0]
-A INPUT -j RH-Firewall-1-INPUT
-A FORWARD -j RH-Firewall-1-INPUT
-A RH-Firewall-1-INPUT -i lo -j ACCEPT
-A RH-Firewall-1-INPUT -i eth0 -j ACCEPT
-A RH-Firewall-1-INPUT -p icmp --icmp-type any -j ACCEPT
-A RH-Firewall-1-INPUT -p 50 -j ACCEPT
-A RH-Firewall-1-INPUT -p 51 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp -m tcp -s 65.131.121.72 -i eth1 --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp -m tcp -s 65.131.121.75 -i eth1 --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -p tcp -m tcp -s 65.131.121.76 -i eth1 --dport 25 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 110 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 143 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 587 -j ACCEPT
-A RH-Firewall-1-INPUT -j REJECT --reject-with icmp-host-prohibited
COMMIT

The system has two interfaces, eth0 (internal) and eth1 (external).

Thanks in advance to anybody that can help me.

Best Regards,

Nick

Nathanael · 04-03-2007, 10:59 AM

"help with iptables" does not really describe the problem...
try to be more specific next time as more people tend to read more descriptive titles :-)

you are looking for the target dnat :-)

iptables -t nat -A -p tcp -i eth1 --dport 25 -j DNAT --to-destination <ipaddr>

search for DNAT in the man pages :-)

technick · 04-03-2007, 01:20 PM

Sorry about the post title, anyways, I tried your suggestion and all I get back is "bad argument 'tcp'".

timdsmith · 04-03-2007, 01:53 PM

You will need two lines to be able to do what you want to do...

Code:

/sbin/iptables -A FORWARD -p tcp -d 192.168.1.100 --dport 25 -j ACCEPT
/sbin/iptables -A PREROUTING -t nat -p tcp -i $EXTIF -d $EXTIP --dport 25 -j DNAT --to 192.168.1.100:25

$EXTIF will be your ethernet card connected to the internet, eth0, eth1, etc...$EXTIP will be your external ip address...If you have a script and want to determine your ip you can get it with this command:

Code:

/sbin/ifconfig eth0 |  /bin/grep 'inet addr' | /usr/bin/awk '{print $2}' | /bin/sed -e 's/.*://'

192.168.1.100 is the ip of the computer you want requests forwarded to.